It wasnt the battery, sadly. Some Fairphone3 have the issue, that their mainboard(?) is breaking down after a couple of years without any clear reason - and I was the lucky one. Its the only “unreplaceble” part and kind of phone breaker. Sure, you can replace this too, but it would somheow be like buying a new phone. At this point, the fairphone 4 was alreasy released, so I took the upgrade. Fairphone 4 and 5 have now a waranty of 5 instead of 2 years. So at least on this point ill be save for 3 years more :>